Antenna diversity switching device with switching circuits between the receiver terminal and each antenna

ABSTRACT

An antenna switching device of the present invention includes: a transmitting terminal for receiving a transmitting signal; a first antenna terminal connected to a first antenna; a second antenna terminal connected to a second antenna; a receiving terminal for outputting receiving signals received at the first antenna terminal and the second antenna terminal; a selecting unit for selecting one mode among a first mode for outputting a signal corresponding to the transmitting signal to the first antenna terminal, a second mode for outputting the receiving signal received at the first antenna to the receiving terminal, and a third mode for outputting the receiving signal received at the second antenna to the receiving terminal.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an antenna switching device or anantenna switching duplexer for use in a radio communication apparatussuch as a digital portable radio telephone performing radiocommunication by selectively connecting an antenna to a transmitter or areceiver, i.e., using a TDMA (time division multiple access) system.

2. Description of the Related Art

In the past, a single-pole double throw switch (hereinafter, referred toas an SPDT switch) has been used for selectively connecting an antennato a transmitter or a receiver in mobile communication equipment. Anexemplary conventional SPDT switch will be described with reference toFIG. 7.

FIG. 7 shows a circuit configuration of a conventional SPDT switch. TheSPDT switch includes a first signal terminal 601, a second signalterminal 602, a third signal terminal 603, a control terminal 604, PIN(positive-intrinsic-negative) diodes 605 and 606, a strip line 607having a 1/4 wavelength of a signal frequency, a resistor 608, aninductor 609, and capacitors 610, 611, 612, and 613. The resistor 608,the inductor 609, and the capacitor 611 constitute a control circuit.One end of the capacitor 611 is connected to one end of the inductor609, and the other end of the capacitor 611 is grounded. The capacitors610, 612, and 613 function as direct-current blocking capacitors.

This conventional SPDT switch is used as a transmit-receive switch. Theoperation of the SPDT switch as a transmit-receive switch will bedescribed. It is noted that in this case, the first, second, and thirdsignal terminals 601, 602, and 603 work as a transmitting terminal, anantenna terminal, and a receiving terminal, respectively.

When a signal is transmitted from the antenna terminal, a positivevoltage is applied to the control terminal 604. A control current flowsthrough the control terminal 604, the resistor 608, the inductor 609,the PIN diode 605, the strip line 607, and the PIN diode 606 in thisorder. Associated with the flow of the control current, the impedancebetween an anode and a cathode of the PIN diodes 605 and 606 becomeslow. The low impedance state between the anode and the cathode of thePIN diode 606 renders an electrical potential at one end of the stripline 607 connected to the anode of the PIN diode 606 at almost a groundlevel. This causes the impedance at the other end of the strip line 607with respect to the antenna terminal 602 to be remarkably increased. Asa result, a transmitting signal input from the transmitting terminal 601is outputted from the antenna terminal 602 but is not outputted from thereceiving terminal 603.

When a signal is received from the antenna terminal, no voltage isapplied to the control terminal 604. Since no control current flows, theimpedance between the anode and the cathode of the respective PIN diodes605 and 606 becomes high. As a result, a receiving signal input from theantenna terminal 602 is outputted from the receiving terminal 503 but isnot outputted from the transmitting terminal 601.

As described above, an antenna is selectively connected to a transmitteror a receiver by using the conventional SPDT switch.

However, in the case where the above-mentioned SPDT switch is applied toa diversity system capable of providing superior receivingcharacteristics in mobile radio communication, there arise problems.That is, since the diversity system utilizes two antennas, two SPDTswitches are required for the system. In this case, the number ofcomponents included in a communication apparatus is increased, making itdifficult to realize a miniaturized apparatus and rendering the loss ofsignal in the apparatus large.

SUMMARY OF THE INVENTION

The antenna switching device of this invention, comprises:

a transmitting terminal for receiving a transmitting signal;

a first antenna terminal connected to a first antenna;

a second antenna terminal connected to a second antenna;

a receiving terminal for outputting receiving signals received at thefirst antenna terminal and the second antenna terminal;

selecting means for selecting one mode among a first mode for outputtinga signal corresponding to the transmitting signal to the first antennaterminal, a second mode for outputting the receiving signal received atthe first antenna to the receiving terminal, and a third mode foroutputting the receiving signal received at the second antenna to thereceiving terminal.

According to the antenna switching device of the present invention, twoSPDT switches are not required for reception and transmission, and atleast one PIN diode can be omitted in a signal path from the firstantenna terminal to the receiving terminal, compared with a switchingdevice using two SPDT switches.

Thus, the invention described herein makes possible the advantage ofproviding a low-loss miniaturized antenna switching device capable ofselectively connecting an antenna to a transmitter or a receiver andswitching antennas for receiving a signal.

This and other advantages of the present invention will become apparentto those skilled in the art upon reading and understanding the followingdetailed description with reference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of an antenna switching device of Example 1according to the present invention.

FIG. 2 is a plan view of the antenna switching device of Example 1according to the present invention.

FIG. 3 is a circuit diagram of an antenna switching device of Example 2according to the present invention.

FIG. 4 is a circuit diagram of an antenna switching device of Example 3according to the present invention.

FIG. 5 is a circuit diagram of an antenna switching device of Example 4according to the present invention.

FIG. 6 is a circuit diagram of an antenna switching device of Example 5according to the present invention.

FIG. 7 is a circuit diagram of a conventional SPDT switch.

FIG. 8 is a circuit diagram of another embodiment of the secondswitching circuit for use in the antenna switching device of Example 1.

FIG. 9 is a circuit diagram of another embodiment of the secondswitching circuit for use in the antenna switching device of Example 3.

FIG. 10 is a circuit diagram of another embodiment of the thirdswitching circuit for use in the antenna switching device of Example 3.

FIG. 11 is a circuit diagram of another embodiment of the secondswitching circuit for use in the antenna switching device of Example 5.

FIG. 12 is a circuit diagram of another embodiment of the firstswitching circuit for use in the antenna switching device of Example 4.

FIG. 13 is a circuit diagram of another embodiment of the firstswitching circuit for use in the antenna switching device of Example 5.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, the present invention will be described by way ofillustrative examples with reference to the drawings.

EXAMPLE 1

Referring to FIGS. 1 and 2, an antenna switching device of Example 1according to the present invention includes a transmitting terminal 101for receiving a transmitting signal, a first antenna terminal 102connected to a first antenna, a second antenna terminal 103 connected toa second antenna, a receiving terminal 104 for outputting a receivingsignal received from the first antenna terminal 102 and the secondantenna terminal 103, a first control terminal 105 for receiving a firstcontrol signal, and a second control terminal 106 for receiving a secondcontrol signal. The antenna switching device further includes a firstswitching circuit 147, a second switching circuit 148, a third switchingcircuit 149, a low-pass filter 145 electrically connected between thetransmitting terminal 101 and the first switching circuit 147, and aband-pass filter 146 electrically connected between the second switchingcircuit 148 and the receiving terminal 104. The first switching circuit147 has an input terminal 162 electrically connected to the transmittingterminal 101 and a first output terminal 164 electrically connected tothe first antenna terminal 102. The second switching circuit 148 has asecond input terminal 166 electrically connected to the first antennaterminal 102, and a second output terminal 168 electrically connected tothe receiving terminal 104. The third switching circuit 149 has a thirdinput terminal 170 electrically connected to the second antenna terminal103 and a third output terminal 172 electrically connected to thereceiving terminal 104.

The first switching circuit 147, the second switching circuit 148, endthe third switching circuit 149 constitute selecting means. Theselecting means selects one mode among a first mode for outputting asignal corresponding to the transmitting signal to the first antennaterminal 102, a second mode for outputting a signal received at thefirst antenna to the receiving terminal 104, and a third mode foroutputting a signal received at the second antenna to the receivingterminal 104. The selecting means selects the first mode when a voltagehaving the first level is supplied to the first control terminal 105,selects the third mode when a voltage having the second level issupplied to the second control terminal 106, end selects the second modewhen a voltage is not supplied to the first and second control terminals105 and 106.

In FIGS. 1 and 2, the reference numerals 108, 109, and 110 denote PINdiodes; 114 and 115 one end short-circuited dielectric coaxialresonators (dielectric resonators); 116, 117, 118, 119, 120, 121, 122,123, and 124 inductors; 126 and 127 resistors; 129, 130, 131, 132, 133,134, 135, 136, 137, 138, 139, 140, 141, 142, 143, and 144 capacitors;150, 151, 152, 153, 154, and 155 ground terminals; and 156 a groundelectrode pattern.

The inductors 116 and 117 end capacitors 129, 130, and 131 constitutethe low-pass filter 145 which is a transmission filter. The low-passfilter 145 allows a signal having a frequency within and lower than atransmitting frequency band to pass therethrough and has high impedancewith respect to higher harmonics of the transmitting signal. Thedielectric resonators 114 and 115, the inductor 124, and the capacitors142, 143, and 144 constitute the band-pass filter 146 which is areception filter. The band-pass filter 146 allows a signal having afrequency within a receiving frequency band to pass therethrough and hashigh impedance with respect to signals having other frequencies.

The band-pass filter 146 includes a circuit portion electricallyconnected between the second antenna terminal 103 and the receivingterminal 104, and the dielectric resonators 114 and 115 each of whichelectrically connected between the receiving terminal 104 and theground. In this example, the capacitors 142, 143, and 144, and theinductor 124 constitute the circuit portion of the band-pass filter 146.

The PIN diode 108, the inductor 119, and the capacitor 134 constitutethe first switching circuit 147.

The PIN diode 109, the inductors 120 and 121, and the capacitors 136,137, and 141 constitute the second switching circuit 148. In the secondswitching circuit 148, a forward voltage is applied to the PIN diode 109to bring a low impedance state between an anode and a cathode thereofand a contact point between the inductors 120 and 121 is almostgrounded. As a result, the inductor 120 and the capacitor 136 and theinductor 121 and the capacitor 141 respectively resonate in parallelwith each other to bring extremely high impedance between the secondinput terminal 166 and the second output terminal 168.

The PIN diode 110, the inductor 122, and the capacitor 140 constitutethe third switching circuit 149. A circuit constituted by the inductor118, the resistor 126, and the capacitors 132 and 133 functions as aninput matching circuit as well as a control circuit for the firstswitching circuit 147. A circuit constituted by the inductor 123, theresistor 127, the capacitors 138 and 139 function as an input matchingcircuit as well as a control circuit for the third switching circuit149.

The capacitor 137 having appropriate capacitance contributes to theimprovement of the impedance matching at the first antenna terminal 102and the receiving terminal 104 when a signal is received at the firstantenna terminal 102.

The low-pass filter 145 which is a transmission filter removes a higherharmonics component from the transmitting signal, thereby preventing thehigher harmonics from being irradiated. A direct-current blockingcapacitor is not required for a connecting point between the band-passfilter 146 and the third switching circuit 149 by using the capacitor142 as an input coupling element of the band-pass filter 146. Theband-pass filter 146 which is a reception filter removes unwanted signalcomponents from the receiving signal. In the case where the band-passfilter 146 is made a polarized band-pass filter, the number ofresonators of the band-pass filter 146 can be decreased compared withthe case where the band-pass filter 146 is not polarized.

In the second switching circuit 148, one end of the inductor 120 and oneend of the capacitor 136 are electrically connected to the second inputterminal 166, and the other end of the capacitor 136 is grounded. Theother end of the inductor 120 is electrically connected to one end ofthe PIN diode 109 and one end of the inductor 121. The other end of thePIN diode 109 is grounded. The other end of the inductor 121 and one endof the capacitor 141 are electrically connected to the second outputterminal 168, and the other end of the capacitor 141 is grounded. Thesecond switching circuit 148 can be miniaturized with such aconfiguration.

As shown in FIG. 2, the antenna switching device of this exampleincludes a substrate 100 on which elements of the device are formed. Thesubstrate 100 includes a first portion on which the low-pass filter 145is formed, a second portion on which the circuit portion of theband-pass filter 146 is formed, and a third portion on which thedielectric resonators 114 and 118 are formed. The third portion of thesubstrate 180 is interposed between the first portion and the secondportion. The transmitting terminal 101 is formed beside the low-passfilter 145, the ground terminal 155 and the ground electrode pattern 156are formed beside the dielectric resonators 114 and 115, and thereceiving terminal 104 is formed beside the circuit portion of theband-pass filter 146, on the substrate 100. Since the ground electrodepattern 156 connected to the ground terminal 155 and the dielectricresonators 114 and 115 are provided between the transmitting terminal101 and the receiving terminal 104, the deterioration of the isolationbetween the transmitting terminal 101 and the receiving terminal 104caused by the electromagnetic coupling between the low-pass filter 145and the band-pass filter 146 can be prevented.

In the first switching circuit 147, a series circuit of the inductor 119and the capacitor 134 is connected in parallel with the diode 108. Thepurpose of this connection is to increase the impedance between thefirst input terminal 162 and the first output terminal 164 of the firstswitching circuit 147 when the first switching circuit 147 isinterrupted. In the case where the inductor 119 and the capacitor 134are removed from the first switching circuit 147, the impedance betweenthe first input terminal 162 and the first output terminal 164 isdecreased. However, even in this case, the same function as the abovecan be realized.

Even though a quarter-wave transmission section is substituted foreither the inductor 120 and the capacitor 136 or the inductor 121 andthe capacitor 141, or for both of them as shown in FIG. 8, the samefunction can be realized. In this case, a specific effect can beobtained, that is, the loss of the receiving signal at a time when thereceiving signal is received at the first antenna terminal 102 isreduced.

Hereinafter, the operation of the antenna switching device of thepresent example will be described.

When the transmitting signal is transmitted (first mode), a positivevoltage is applied to the first control terminal 105, and no voltage isapplied to the second control terminal 106. A control current flowsthrough the first control terminal 105, the resistor 126, the inductor118, the first switching circuit 147, the inductor 120, and the PINdiode 109 in this order. Associated with the flow of the controlcurrent, the impedance between the first input terminal 162 and thefirst output terminal 164 of the first switching circuit 147 and betweenan anode and a cathode of the PIN diode 109 becomes low. The lowimpedance state between the anode and the cathode of the PIN diode 109renders a contact point between the inductors 120 and 121 almostgrounded. Because of this,, the inductor 120 and the capacitor 136 andthe inductor 121 and the capacitor 141 respectively resonate in parallelwith each other to bring extremely high impedance between the secondinput terminal 166 and the second output terminal 168 in the secondswitching circuit 148. Consequently, the transmitting signal input fromthe transmitting terminal 101 is outputted from the first antennaterminal 102 but is not outputted from the other terminals.

The antenna switching device of the present example has two receivingmodes in addition to the above-mentioned transmitting mode. Thesereceiving modes are classified into that receiving a signal from thefirst antenna terminal 102 (second mode) and that receiving a signalfrom the second antenna terminal 103 (third mode).

First, the operation of the mode for receiving a signal from the firstantenna terminal 102 will be described.

When a signal is received from the first antenna terminal 102, novoltage is applied to the first control terminal 105 and the secondcontrol terminal 105. Since no control current flows through the device,the impedance between the first input terminal 162 and the first outputterminal 164 of the first switching circuit 147, between the third inputterminal 170 and the third output terminal 172 of the third switchingcircuit 149, and between the anode and the cathode of the PIN diode 109becomes high. As a result, the receiving signal input from the firstantenna terminal 102 is outputted from the receiving terminal 104 but isnot outputted from the other terminals.

Next, the operation of the mode receiving a signal at the second antennaterminal 103 will be described.

When a signal is received from the second antenna terminal 103, novoltage is applied to the first control terminal 105, and a positivevoltage is applied to the second control terminal 106. A control currentflows through the second control terminal 105, the resistor 127, theinductor 123, the third switching circuit 149, the inductor 121, and thePIN diode 109 in this order. Associated with the flow of the controlcurrent, the impedance between the first input terminal 162 and thefirst output terminal 164 in the first switching circuit 147 becomeshigh, and the impedance between the third input terminal 170 and thethird output terminal 172 of the third switching circuit 149 and betweenthe anode and the cathode of the PIN diode 109 becomes low. The lowimpedance state between the anode and the cathode of the PIN diode 109renders the contact point of the inductors 120 and 121 almost grounded.Because of this, the inductor 120 and the capacitor 136 and the inductor121 and the capacitor 141 resonate in parallel with each other to bringextremely high impedance between the second input terminal 166 and thesecond output terminal 168 in the second switching circuit 148.Consequently, the receiving signal input from the second antennaterminal 103 is outputted from the receiving terminal 104, but is notoutputted from the other terminals.

In the antenna switching device of Example 1 according to the presentinvention having the above-mentioned construction and function, at leastone PIN diode element can be omitted in a signal path from the firstantenna terminal 102 to the receiving terminal 104, unlike the switchingdevice using two SPDT switches. Furthermore, according to the presentinvention, a low-loss miniaturized antenna switching device capable ofselectively connecting an antenna to a transmitter or a receiver,removing unwanted signal components from the receiving signal bysuppressing higher harmonics thereof, and switching antennas forreception can be obtained.

EXAMPLE 2

Referring to FIG. 3, an antenna switching device of Example 2 accordingto the present invention includes a transmitting terminal 201 forreceiving a transmitting signal, a first antenna terminal 202 connectedto a first antenna, a second antenna terminal 203 connected to a secondantenna, a receiving terminal 204 for outputting a receiving signalreceived from the first antenna terminal 202 and the second antennaterminal 203, a first control terminal 205 for receiving a first controlsignal, and a second control terminal 206 for receiving a second controlsignal. The antenna switching device further includes a first switchingcircuit 247, a second switching circuit 248, and a third switchingcircuit 249, a low-pass filter 245 electrically connected between thetransmitting terminal 201 and the first switching circuit 247, and aband-pass filter 246 electrically connected between a second switchingcircuit 248 and the receiving terminal 204. The first switching circuit247 has a first input terminal 262 electrically connected to hetransmitting terminal 201 and a first output terminal 264 electricallyconnected to the first antenna terminal 202. The second switchingcircuit 248 has a second input terminal 266 electrically connected tothe first antenna terminal 202, and a second output terminal 268electrically connected to the receiving terminal 204. The thirdswitching circuit 249 has a third input terminal 270 electricallyconnected to the second antenna terminal 203 and a third output terminal272 electrically connected to the receiving terminal 204.

The first switching circuit 247, the second switching circuit 248, andthe third switching circuit 249 constitute selecting means. Theselecting means selects one mode among a first mode for outputting asignal corresponding to the transmitting signal to the first antennaterminal 202, a second mode for outputting a signal received at thefirst antenna to the receiving terminal 204, and a third mode foroutputting a signal received at the second antenna to the receivingterminal 204. The selecting means selects the first mode when a voltagehaving a predetermined level is supplied to the first control terminal205, selects the third mode when voltages having predetermined levelsare supplied to the first and second control terminals 205 and 206, andselects the second mode when a voltage is not supplied to the first andsecond control terminals 205 and 206.

In FIG. 3, the reference numerals 208, 209, and 210 denote PIN diodes;214 and 215 one end short-circuited dielectric coaxial resonators(dielectric resonators); 216, 217, 218, 219, 220, 221, 222, 223, and 224inductors; 226 and 227 resistors; 229, 230, 231, 232, 233, 234, 235,236, 237, 238, 239, 240, 241, and 242 capacitors.

The inductors 216 and 217 and capacitors 229, 230, and 231 constitutethe low-pass filter 245. The low-pass filter 245 allows a signal havinga frequency within and lower than a transmitting frequency band to passtherethrough and has high impedance with respect to higher harmonics ofthe transmitting signal. The dielectric resonators 214 and 215, theinductor 224, and the capacitors 240, 241, and 242 constitute theband-pass filter 246. The band-pass filter 246 allows a signal having afrequency within a receiving frequency band to pass therethrough and hashigh impedance with respect to signals having other frequencies.

The PIN diode 208, the inductor 219, and the capacitor 234 constitutethe first switching circuit 247.

The PIN diode 209, the inductor 220, and the capacitor 236 constitutethe second switching circuit 248. In the second switching circuit 248,when a forward voltage is applied to the PIN diode 209 to bring a lowimpedance state between an anode and a cathode thereof, the inductor 220and the capacitor 236 resonate in parallel with each other to bringextremely high impedance between the second input terminal 266 and thesecond output terminal 268. When a forward voltage is not applied to thePIN diode 209 and the impedance between the anode and the cathode of thePIN diode 209 becomes high, the inductor 220 and the capacitor 236 donot resonate in parallel with each other to bring a low impedance statebetween the second input terminal 266 and the second output terminal268. In the second switching circuit 248, a series circuit of theinductor 220 and the PIN diode 299 is electrically connected in parallelwith the capacitor 236 between the second input terminal 266 and thesecond output terminal 268. This construction provides an effect ofreducing the loss of the receiving signal when the receiving signal isreceived at the first antenna terminal 202.

The PIN diode 210, the inductor 221, and the capacitor 239 constitutethe third switching circuit 249. A circuit constituted by the inductor218, the resistor 226, and the capacitors 232 and 233 functions as aninput matching circuit as well as a control circuit for the firstswitching circuit 247. A circuit constituted by the inductor 222, theresistor 227, the capacitors 237 and 238 function as an input matchingcircuit as well as a control circuit for the third switching circuit249.

Hereinafter, the operation of the antenna switching device of thepresent example will be described.

When the transmitting signal is transmitted (first mode), a positivevoltage is applied to the first control terminal 205, and no voltage isapplied to the second control terminal 206. A control current flowsthrough the first control terminal 205, the resistor 226, the inductor218, the first switching circuit 247, the second switching circuit 248,and the inductor 223 in this order. Associated with the flow of thecontrol current, the impedance between the first input terminal 262 andthe first output terminal 264 of the first switching circuit 247 becomeslow and the impedance between the second input terminal 266 and thesecond output terminal 268 of the second switching circuit 248 becomeshigh. Consequently, the transmitting signal input from the transmittingterminal 201 is outputted from the first antenna terminal 202 but is notoutputted from the other terminals.

The antenna switching device of the present example has two receivingmodes in addition to the above-mentioned transmitting mode. Thesereceiving modes are classified into that receiving a signal from thefirst antenna terminal 202 (second mode) and that receiving a signalfrom the second antenna terminal 203 (third mode).

First, the operation of the mode for receiving a signal from the firstantenna terminal 202 will be described.

When a signal is received from the first antenna terminal 202, novoltage is applied to the first control terminal 205 and the secondcontrol terminal 206. Since no control current flows through the device,the impedance between the first input terminal 262, and the first outputterminal 264 of the first switching circuit 247 and between the thirdinput terminal 270 and the third output terminal 272 of the thirdswitching circuit 249 becomes high. Simultaneously, the impedancebetween the second input terminal 266 and the second output terminal 268of the second switching circuit 248 becomes low. As a result, thereceiving signal input from the first antenna terminal 202 is outputtedfrom the receiving terminal 204 but is not outputted from the otherterminals.

Next, the operation of the mode for receiving a signal at the secondantenna terminal 203 will be described.

When a signal is received from the second antenna terminal 203, apositive voltage is applied to the first control terminal 205 and thesecond control terminal 206. A control current flows through the firstcontrol terminal 205, the resistor 226, the inductor 218, the firstswitching circuit 247, the second switching circuit 248, and theinductor 223 in this order. Simultaneously, the control current flowsthrough the second control terminal 206, the resistor 227, the inductor222, the third switching circuit 249, and the inductor 223 in thisorder. Associated with the flow of the control current, the impedancebetween the first input terminal 262 and the first output terminal 264of the first switching circuit 247 and between the third input terminal270 and the third output terminal 272 of the third switching circuit 249becomes low, and the impedance between the second input terminal 266 andthe second output terminal 268 of the second switching circuit 248becomes high. Consequently, the receiving signal input from the secondantenna terminal 203 is outputted from the receiving terminal 204, butis not outputted from the other terminals.

In the antenna switching device of Example 2 according to the presentinvention has the same function as that of Example 1. However, in thepresent example, the number of elements between the first antennaterminal and the receiving terminal is less than that of Example 1.Thus, the loss of the receiving signal in the circuit can be furtherminimized.

EXAMPLE 3

Referring to FIG. 4, an antenna switching device of Example 3 accordingto the present invention includes a transmitting terminal 301 forreceiving a transmitting signal, a first antenna terminal 302 connectedto a first antenna, a second antenna terminal 303 connected to a secondantenna, a receiving terminal 304 for outputting a receiving signalreceived from the first antenna terminal 302 and the second antennaterminal 303, a first control terminal 305 for receiving a first controlsignal, a second control terminal 306 for receiving a second controlsignal, a third control terminal 307 for receiving a third controlsignal, a fourth control terminal 308 for receiving a fourth controlsignal, and a fifth control terminal 309 for receiving a fifth controlsignal. The antenna switching device further includes a first switchingcircuit 342, a second switching circuit 343, a third switching circuit344, a low-pass filter 340 electrically connected between thetransmitting terminal 301 and the first switching circuit 342, and aband-pass falter 341 electrically connected between the second switchingcircuit 343 and the receiving terminal 304. The first switching circuit342 has a first input terminal 362 electrically connected to thetransmitting terminal 301 end a first output terminal 364 electricallyconnected to the first antenna terminal 302. The second switchingcircuit 343 has a second input terminal 366 electrically connected tothe first antenna terminal 302, and a second output terminal 368electrically connected to the receiving terminal 304. The thirdswitching circuit 344 has a third input terminal 370 electricallyconnected to the second antenna terminal 303 and a third output terminal372 electrically connected to the receiving terminal 304.

The first switching circuit 342, the second switching circuit 343, andthe third switching circuit 344 constitute selecting means. Theselecting means selects one mode among a first mode for outputting asignal corresponding to the transmitting signal to the first antennaterminal 302, a second mode for outputting a signal received at thefirst antenna to the receiving terminal 304, and a third mode foroutputting a signal received at the second antenna to the receivingterminal 304. The selecting means selects the first mode when voltageshaving predetermined levels are respectively applied to the first,third, and fifth control terminals 305, 307, and 309, selects the secondmode when voltages having predetermined levels are respectively appliedto the second and fifth control terminals 306 and 309, and selects thethird mode when voltages having predetermined levels are respectivelyapplied to the third and fourth control terminals 307 and 308.

In FIG. 4, the reference numeral 310 denotes a PIN diode; 311, 312, 313,and 314 field effect transistors (FETs); 315 and 316 one endshort-circuited dielectric coaxial resonators (dielectric resonators);317, 318, 319, 320, 321, and 322 inductors; 323 a strip line; 324, 325,326, 327, and 328 resistors; and 329, 330, 331, 332, 333, 334, 335, 336,337, 338, and 339 capacitors.

The strip line 323 has a characteristic impedance of about 50Ω, and hasa length of 1/4 wavelength of a signal having a transmitting frequency.

The inductors 317 and 318 and capacitors 329, 330, and 331 constitutethe low-pass filter 340. The low-pass filter 340 allows a signal havinga frequency within and lower than a transmitting frequency band to passtherethrough and has high impedance with respect to higher harmonics ofthe transmitting signal. The dielectric resonators 315 and 316, theinductor 322, and the capacitors 337, 338, and 339 constitute theband-pass filter 341. The band-pass filter 341 allows a signal having afrequency within a receiving frequency band to pass therethrough and hashigh impedance with respect to signals having the other frequencies.

The PIN diode 310, the inductor 320, and the capacitor 334 constitutethe first switching circuit 342. A circuit constituted by the inductor319, the resistor 324, the capacitors 332 and 333 functions as an inputmatching circuit as well as a control circuit for the first switchingcircuit 342.

The FETs 311 and 312, the strip line 323, the control terminals 306 and307, and the resistors 325 and 326 constitute the second switchingcircuit 343. In the case where a voltage is not applied to the secondcontrol terminal 306, the impedance between the drain and the source ofthe FET 311 becomes low, and one end of the strip line 323 is almostgrounded, the second switching circuit 343 has an extremely highimpedance with respect to the first antenna terminal 302.

The FETs 313 and 314, the control terminals 308 and 309, the inductor321, and the resistors 327 and 328 constitute the third switchingcircuit 344. The resistance between the drain and the source of the FET313 at low impedance is almost equal to a matched load.

In the first switching circuit 342, the PIN diode 310 allows a largesignal to be transmitted therethrough with less distortion than when anFET is used.

Because of the construction in which the second switching circuit 343and the third switching circuit 344 are respectively formed using a FET,electric power consumed by the antenna switching device becomes nearlyzero at a time when a signal is received.

In the second switching circuit 343, one end of the strip line 323 iselectrically connected to the second Input terminal 366, and the otherend of the strip line 323 is electrically connected to the drain of theFET 311 and the drain of the FET 312. The source of the FET 311 isgrounded and the source of the FET 312 is electrically connected to thesecond output terminal 368. Because of this configuration of the secondswitching circuit 343, electric power consumed by the second switchingcircuit 343 becomes almost zero and the deterioration of the isolationbetween the first antenna terminal 302 and the receiving terminal 304 isprevented at a time when a transmitting power is large.

In the third switching circuit 344, the resistance between a drain and asource of the FET 313 at low impedance is almost equal to a matchedload. The drain of the FET 313 and the drain of the FET 314 areelectrically connected to the third input terminal 370, the source ofthe FET 313 is grounded, and the source of the FET 314 is electricallyconnected to the third output terminal 372. Because of thisconfiguration of the third switching circuit 344, when the impedancebetween the drain and the source of the FET 313 becomes low and theimpedance between the drain and the source of the FET 314 becomes high,the second antenna terminal 303 becomes substantially connected to amatched load. Thus, when the transmitting signal is transmitted and whenthe receiving signal is received from the first antenna terminal 302,the interference between the first antenna connected to the firstantenna terminal 302 and the second antenna connected to the secondantenna terminal 303 can be prevented.

In the second switching circuit 343, the strip line 323 can be replacedby an inductor one end of which is connected the second input terminal366 and the other end of which is connected to the drain of the FET 311and a capacitor one end of which is connected to the second inputterminal 366 and the other end of which is grounded as shown in FIG. 9.In this case, the same effects as those obtained when the strip line 323is used can be obtained, and simultaneously the antenna switching devicecan be readily miniaturized.

The same effects can also be obtained when one end of a resistor havinga resistance nearly equal to the matched load is connected to the sourceof the FET 313 and the other end of the resistor is grounded, as shownin FIG. 10, instead of making the resistance between the drain and thesource of the FET 313 at low impedance almost equal to the matched load.

In the case where the resistance between the drain and the source of theFET 313 at low impedance is made almost zero, although the secondantenna terminal 303 will not be connected to a matched load, theisolation of the third switching circuit 344 at a time when the thirdswitching circuit 344 is interrupted is increased. It is noted that whenthe FET 313 is removed from the third switching circuit 344, even thoughthe above-mentioned effects are lost, the third switching circuit 344retains required functions.

In the third switching circuit 344, one end of the inductor 321 isconnected to the drain of the FET 314 and the other end of the inductor321 is connected to the source of the FET 314. Because of thisconfiguration, the impedance between the third input terminal 370 andthe third output terminal 372 of the third switching circuit 344 can beincreased at a time when the third switching circuit 344 is interrupted.The inductor 321 can be removed from the third switching circuit 344.However, even though the inductor 321 is removed and the impedancebetween the third input terminal 370 and the third output terminal 372is decreased, the same function as that of the present example can berealized.

Hereinafter, the operation of the antenna switching device of thepresent example will be described.

When the transmitting signal is transmitted (first mode), a positivevoltage is applied to the first control terminal 305, a negative voltageequal to or lower than a pinch off voltage is applied to the third andfifth control terminals 307 and 309, and no voltage is applied to thesecond and fourth control terminals 306 and 308. A control current flowsthrough the first control terminal 305, the resistor 324, the inductor319, the first switching circuit 342, the strip line 323, and the FET311 in this order. When the control voltages are applied to therespective control terminals, the impedance between the first inputterminal 362 and the first output terminal 364 of the first switchingcircuit 342 and between the drain and the source of the FETs 311 and 313becomes low, and the impedance between the drain and the source of theFETs 312 and 314 becomes high. The low impedance state between the drainand the source of the FET 311 renders one end of the strap line 323almost grounded. This causes the second switching circuit 343 to haveextremely high impedance with respect to the first antenna terminal 302.As a result, the transmitting signal input from the transmittingterminal 301 is outputted from he first antenna terminal 302 but is notoutputted from the other terminals. Furthermore at this time, theimpedance between the drain and the source of the FET 313 becomes lowand the impedance between the drain and the source of the FET 314becomes high. Thus, the second antenna terminal 303 becomessubstantially connected to a matched load.

The antenna switching device of the present example has two receivingmodes in addition to the above-mentioned transmitting mode. Thesereceiving modes are classified into that receiving a signal from thefirst antenna terminal 302 (second mode) and that receiving a signalfrom the second antenna terminal 303 (third mode).

First, the operation of the mode for receiving a signal at the firstantenna terminal 302 will be described.

When a signal is received from the first antenna terminal 302, anegative voltage equal to or less than a pinch off voltage is applied tothe second and fifth control terminals 306 and 309. At this time, novoltage is applied to the first, third, and fourth control terminals305, 307, and 308. When such a control voltage is applied to therespective control terminals, the impedance between the first inputterminal 362 and the first output terminal 364 of the first switchingcircuit 342 end between the drain and the source of the FETs 311 and 314becomes high, and the impedance between the source and the drain of theFETs 312 and 313 becomes low. Consequently, the receiving signal inputfrom the first antenna terminal 302 is outputted from the receivingterminal 304 but is not outputted from the other terminals. When theimpedance between the drain and the source of the FET 313 becomes lowand the impedance between the drain and the source of the FET 314becomes high, the second antenna terminal 303 becomes substantiallyconnected to a matched load.

Next, the operation of the mode for receiving a signal at the secondantenna terminal 303 will be described.

When a signal is received from the second antenna terminal 303, anegative voltage equal to or less than a pinch off voltage is applied tothe third control terminal 307 and the fourth control terminal 308. Atthis time, no voltage is applied to the first, second and fifth controlterminals 305, 306, end 309. When such a control voltage is applied tothe respective control terminals, the impedance between the first inputterminal 362 and the first output terminal 364 of the first switchingcircuit 342 and between the drain end the source of the FETs 312 and 313becomes high, and the impedance between the drain and the source of theFETs 311 and 314 becomes low. As a result, the receiving signal inputfrom the second antenna terminal 303 is outputted from the receivingterminal 304 but is not outputted from the other terminals.

As described above, the antenna switching device in Example 3 accordingto the present invention has the same function as that of Example 1according to the present invention, and in addition, the antennaswitching device in Example 3 consumes almost no electric power whenreceiving a signal.

EXAMPLE 4

Referring to FIG. 5, an antenna switching device of Example 4 accordingto the present invention includes a transmitting terminal 401 forreceiving a transmitting signal, a first antenna terminal 402 connectedto a first antenna, a second antenna terminal 403 connected to a secondantenna, a receiving terminal 404 for outputting a receiving signalreceived from the first antenna terminal 402 and the second antennaterminal 403, and a control terminal 405 for receiving a control signal.The antenna switching device further includes a first switching circuit447, a second switching circuit 448, a transmission filter 445electrically connected between the transmitting terminal 401 and thefirst switching circuit 447, and a band-pass filter 446 electricallyconnected between the second switching circuit 448 and the receivingterminal 404. The first switching circuit 447 has a first input terminal462 electrically connected to the transmitting terminal 401 and thefirst antenna terminal 402, and a first output terminal 464 electricallyconnected to the receiving terminal 404. The second switching circuit448 has a second input terminal 466 electrically connected to the secondantenna terminal 403 and a second output terminal 468 electricallyconnected to the receiving terminal 404.

The first switching circuit 447 and the second switching circuit 448constitute selecting means. The selecting means selects one mode among afirst mode for outputting a signal corresponding to the transmittingsignal to the first antenna terminal 402, a second mode for outputting asignal received at the first antenna to the receiving terminal 404, anda third mode for outputting a signal received at the second antenna tothe receiving terminal 404. The selecting means selects the first modewhen a positive voltage is supplied to the control terminal 405, selectsthe second mode when no voltage is supplied to the control terminal 405,end selects the third mode when a positive voltage is supplied to thecontrol terminal 405.

In FIG. 5, the reference numerals 408 and 409 denote PIN diodes; 413,414 and 415 one end short-circuited dielectric coaxial resonators(dielectric resonators); 416, 417, 418, 419, 420, and 421 inductors; 426a resistor; 429, 430, 431, 432, 433, 434, 435, 436, 437, 438, 439, 440,and 441 capacitors.

The dielectric resonator 413, the inductors 416 end 417, end thecapacitors 429, 430, end 431 constitute the transmission filter 445. Thetransmission filter 445 allows a signal having a frequency within atransmitting frequency band of the transmitting signal to passtherethrough and has high impedance with respect to the receivingsignal.

The dielectric resonators 414 and 415, the inductor 421, and thecapacitors 439, 440, and 441 constitute the bend-pass filter 446. Theband-pass filter 446 allows a signal having a frequency within areceiving frequency band to pass therethrough and has high impedancewith respect to signals outside of the receiving frequency band.

The PIN diode 408, the inductors 418 and 419, and the capacitors 432,433, 434, and 438 constitute the first switching circuit 447. In thefirst switching circuit 447, when a forward voltage is applied to thePIN diode 408 to bring a low impedance state between an anode and acathode thereof, and a contact point between the inductors 418 and 419is almost grounded, the inductor 418 and the capacitor 432 and theinductor 419 and the capacitor 438 respectively resonate in parallelwith each other. As a result, the impedance between the first inputterminal 462 and the second input terminal 464 becomes extremely high.The capacitor 433 is a DC blocking capacitor.

The PIN diode 409, the inductor 420, and the capacitor 436 constitutethe second switching circuit 448.

The transmission filter 445 is constructed so as to allow a signalhaving a frequency within a transmitting frequency band to passtherethrough and to allow a signal having a frequency within a receivingsignal frequency band to be attenuated. Because of this construction,the transmitting signal input from the transmitting terminal 401 isoutputted from the first antenna terminal 402, and the receiving signalinput from the first antenna terminal 402 is not outputted from thetransmitting terminal 401. By providing the transmission falter 445 withlow-pass characteristics in addition to the above-mentioned transmissioncharacteristics, the transmission filter 445 will be capable ofpreventing the radiation of higher harmonics of the transmitting signal.Since the transmission filter 445 functions as a band-elimination filterusing a dielectric resonator as well as a low-pass filter, the loss ofthe transmitting signal can be reduced and the circuit can beminiaturized, compared with the case of using a band-pass filter as thetransmission filter 445. In the present example, a one-stage notchfilter is used for the transmission filter 445. It is noted that when amulti-stage notch filter is used for the transmission filter 445, higherperformance can be obtained.

The antenna switching device of the present example will have the samefunction as that as described above, even though the first switchingcircuit 447 has the same configuration as that of the second switchingcircuit 248 in Example 2, as shown in FIG. 12.

Hereinafter, the operation of the antenna switching device of thepresent example will be described.

When the transmitting signal is transmitted from the first antenna(first mode), a positive voltage is applied to the control terminal 405.A control current flows through the control terminal 405, the resistor426, the second switching circuit 448, the inductor 419, and the PINdiode 408 in this order. Associated with the flow of the controlcurrent, the impedance between the second input terminal 486 and thesecond output terminal 468 of the second switching circuit 448 andbetween an anode and a cathode of the PIN diode 408 becomes low. The lowimpedance state between the anode and the cathode of the PIN diode 408renders a contact point between the inductors 418 and 419 almostgrounded. This causes the inductor 418 and the capacitor 432 and theinductor 419 and the capacitor 438 to resonate in parallel with eachother. As a result, the impedance between the first input terminal 462and the first output terminal 464 of the first switching circuit 447becomes extremely high. Since the transmission filter 445 allows asignal having a frequency within a transmitting frequency band to passtherethrough, the transmitting signal input from the transmittingterminal 401 passes through the transmission filter 445 and is outputtedfrom the first antenna terminal 402, but is not outputted from the otherterminals.

The antenna switching device of the present example has two receivingmodes in addition to the above-mentioned transmitting mode. Thesereceiving modes are classified into that receiving a signal at the firstantenna terminal 402 (second mode) and that receiving a signal at thesecond antenna terminal 403 (third mode).

First, the operation of the mode for receiving a signal at the firstantenna terminal 402 will be described.

When a signal is received at the first antenna terminal 402, no voltageis applied to the control terminal 405. Since no control current flowsthrough the device, the impedance between the second input terminal 466and the second output terminal 468 of the second switching circuit 448and between the anode and the cathode of the PIN diode 408 becomes high.The impedance of the transmission filter 445 with respect to the firstantenna terminal 402 is extremely high in the receiving frequency band.Consequently, the receiving signal input from the first antenna terminal402 is outputted from the receiving terminal 404 and is not outputtedfrom the other terminals.

Next, the operation of the mode for receiving a signal at the secondantenna terminal 403 will be described.

When a signal is received at the second antenna terminal 403, a positivevoltage is applied to the control terminal 405. A control current flowsthrough the control terminal 405, the resistor 426, the second switchingcircuit 448, the inductor 419, end the PIN diode 408 in this order.Associated with the flow of the control current, the impedance betweenthe second input terminal 466 and the second output terminal 468 of thesecond switching circuit 448 and between the anode and the cathode ofthe PIN diode 408 becomes low. The low impedance state between the anodeand the cathode of the PIN diode 408 renders the contact point betweenthe inductors 418 and 419 almost grounded. This causes the inductor 418and the capacitor 432 and the inductor 419 and the capacitor 438 toresonate in parallel with each other. Consequently, the impedancebetween the input terminal 462 and the output terminal 464 of the firstswitching circuit 447 becomes extremely high. Consequently, thereceiving signal input from the second antenna terminal 403 is outputtedfrom the receiving terminal 404 but is not outputted from the otherterminals.

The antenna switching device in Example 4 according to the presentinvention has the same function as that of Example 1, using less numberof PIN diodes compared with that of Example 1.

EXAMPLE 5

Referring to FIG. 6, an antenna switching device in Example 5 accordingto the present invention includes a transmitting terminal 501 forreceiving a transmitting signal, a first antenna terminal 502 connectedto a first antenna, a second antenna terminal 503 connected to a secondantenna, a receiving terminal 504 for outputting a receiving signalreceived from the first antenna terminal 502 and the second antennaterminal 503, a first control terminal 505 for receiving a first controlsignal, a second control terminal 506 for receiving a second controlsignal, a third control terminal 507 for receiving a third controlsignal, and a fourth control terminal 508 for receiving a fourth controlsignal. The antenna switching device further includes a first switchingcircuit 535, a second switching circuit 536, a transmission filter 533electrically connected between the transmitting terminal 501 and thefirst switching circuit 535, and a band-pass filter 534 electricallyconnected between the second switching circuit 536 and the receivingterminal 504. The first switching circuit 535 has a first input terminal562 electrically connected to the transmitting terminal 501 end thefirst antenna terminal 502, end a first output terminal 564 electricallyconnected to the receiving terminal 504. The second switching circuit536 has a second input terminal 566 electrically connected to the secondantenna terminal 503 and a second output terminal 568 electricallyconnected to the receiving terminal 504.

The first switching circuit 535 and the second switching circuit 536constitute selecting means. The selecting means selects one mode among afirst mode for outputting a signal corresponding to the transmittingsignal to the first antenna terminal 502, a second mode for outputting asignal received from the first antenna to the receiving terminal 504,and a third mode for outputting a signal received from the secondantenna to the receiving terminal 504. The selecting means selects thefirst mode when a voltage having a predetermined level is respectivelysupplied to the second and fourth control terminals 506 and 508, and thefirst and third control terminals 505 and 507 are grounded, selects thesecond mode when voltages having predetermined levels are respectivelysupplied to the first and fourth control terminals 506 and 507 aregrounded, and selects the third mode when voltages having apredetermined levels are supplied to the second and third controlterminals 506 and 507, and the first and fourth control terminals 505and 508 are grounded.

In FIG. 6, the reference numerals 509, 510, 511, and 512 denote FETs;513, 514, and 515 one end short-circuited dielectric coaxial resonators(dielectric resonators); 516, 517, 518, and 519 inductors; 520, 521,522, 523, and 524 resistors; and 525, 526, 527, 528, 529, 530, 531, and532 capacitors.

The dielectric resonator 513, the inductors 516 and 517, and thecapacitors 525, 526, and 527 constitute the transmission filter 533. Thetransmission filter 533 allows a signal having a frequency within atransmitting frequency band to pass therethrough and has high impedancewith respect to the receiving signal.

The dielectric resonators 514 and 515, the inductor 519, and thecapacitors 530, 531, and 532 constitute the band-pass filter 534. Theband-pass filter 534 allows a signal having a frequency within areceiving frequency band and has high impedance with respect to signalshaving a frequency outside of the receiving frequency band.

The FETS 509 and 510, the inductor 518, the resistors 520 and 521, andthe capacitors 528 and 529 constitute the first switching circuit 535.In the first switching circuit 535, when the impedance between a drainand a source of the FET 509 becomes low and one end of the inductor 518is almost grounded, the inductor 518 and the capacitor 528 resonate inparallel with each other. Consequently, the impedance of the firstswitching circuit 535 with respect to the first antenna terminal 502becomes extremely high.

The FETs 511 and 512, the resistors 522, 523, and 524 constitute thesecond switching circuit 536.

In the first switching circuit 535, one end of the inductor 518 and oneend of the capacitor 528 are electrically connected to the first inputterminal 562. The other end of the capacitor 528 is grounded and theother end of the inductor 518 is electrically connected to the drain ofthe FET 509 and the drain of the FET 510. The source of the FET 509 isgrounded, and the source of the FET 510 is electrically connected to thefirst output terminal 564. Because of this configuration of the firstswitching circuit 535, electric power consumed by the first switchingcircuit 535 becomes almost zero and the deterioration of the isolationbetween the first antenna terminal 502 and the receiving terminal 504 isprevented at a time when a transmitting power is large.

In the second switching circuit 536, the resistance of the resistor 522has resistance almost equal to a matched load. Since the second antennaterminal 503 is substantially connected to a matched load at a time whenthe impedance between the drain and the source of the FET 511 is low,unwanted interference between the antenna connected to the first antennaterminal 502 and the antenna connected to the second antenna terminal503 can be prevented, when the transmitting signal is transmitted andthe receiving signal is received at the first antenna terminal 502.

Even though a quarter-wave transmission section is substituted for theinductor 518 and the capacitor 528 as shown in FIG. 13, the same effectsas those of the above can be obtained.

The resistance between the drain and the source of the FET 511 at lowimpedance can be made almost equal to a matched load. Even in this case,the second antenna terminal 503 can be substantially connected to amatched load, when the transmitting signal is transmitted and thereceiving signal is received the first antenna terminal 502. However,the matched load can be more readily regulated when the resistor 522having a resistance almost equal to the matched load is connected to thesource of the FET 511.

When an inductor is connected between the drain of the FET 512 and thesource of the FET 512 in the second switching circuit 536 as shown inFIG. 11, the isolation of the second switching circuit 536 at a timewhen the second switching circuit 536 is interrupted can be increased.In the case where the FET 511 and the resistor 522 are removed from thesecond switching circuit 536, although the above-mentioned effects arelost, the second switching circuit 536 still retains the requiredfunction.

Hereinafter, the operation of the antenna switching device of thepresent example will be described.

When the transmitting signal is transmitted from the first antennaterminal 502 (first mode), a negative voltage equal to or less than apinch-off voltage is applied to the second and fourth control terminals506 and 508, and the first and third control terminals 505 end 507 aregrounded. When the control voltage is applied to the respective controlterminals, the impedance between the drain and the source of the FETs510 and 512 becomes high, and the impedance between the drain and thesource of the FETs 509 and 511 becomes low. The low impedance statebetween the drain and the source of the FET 509 renders one terminal ofthe inductor 518 almost grounded, so that the inductor 518 and thecapacitor 528 resonate in parallel with each other. Consequently, theimpedance of the first switching circuit 535 with respect to the firstantenna terminal 502 becomes extremely high. The transmission filter 533allows a signal having a frequency within transmitting frequency band topass therethrough. Because of this, the transmitting signal input fromthe transmitting terminal 501 passes through the transmission filter 533and is outputted from the first antenna terminal 502 but is notoutputted from the other terminals.

In the first mode, the impedance between the drain and the source of theFET 511 becomes low, the second antenna terminal 503 is connected to theresistor 522 one end of which is grounded, and the impedance between thedrain and the source of the FET 512 becomes high, whereby the secondantenna terminal 503 becomes substantially connected to a matched load.

The antenna switching device of the present example has two receivingmodes in addition to the above-mentioned transmitting mode. Thesereceiving modes are classified into that receiving a signal at the firstantenna terminal 502 (second mode) and that receiving a signal at thesecond antenna terminal 503 (third mode).

First, the operation of the second mode will be described.

When a signal is received at the first antenna terminal 502, a negativevoltage equal to or less than a pinch off voltage is applied to thefirst end fourth control terminals 505 and 508, and the second and thirdcontrol terminals 506 and 507 are grounded. When the control voltage isapplied to the respective control terminals, the impedance between thedrain and the source of the FETs 509 and 512 becomes high, and theImpedance between the drain and the source of the FETs 510 and 511becomes low. As a result, the impedance between the first input terminal562 and the first output terminal 564 of the first switching circuit 535becomes low, and the second switching circuit 536 has high Impedancewith respect to the first switching circuit 535. The impedance of thetransmission filter 533 with respect to the first antenna terminal 502is extremely high within a receiving frequency band. Consequently, thereceiving signal input from the first antenna terminal 502 is outputtedfrom the receiving terminal 504, but is not outputted from the otherterminals.

In the second mode, the impedance between the drain and the source ofthe FET 511 becomes low, the second antenna terminal 503 is connected tothe resistor 522 one end of which is grounded, and the impedance betweenthe drain and the source of the FET 512 becomes high, whereby the secondantenna terminal 503 becomes substantially connected to a matched load.

Next, the operation of the third mode will be described.

When a signal is received at the second antenna terminal 503, a negativevoltage equal to or less than a pinch off voltage is applied to thesecond and third control terminals 506 and 507, and the first and fourthcontrol terminals 505 and 508 are grounded. When the control voltage isapplied to the respective control terminals, the impedance between thedrain and the source of the FETs 510 and 511 becomes high, and theimpedance between the drain and the source of the FETs 509 and 512becomes low. Consequently, the first switching circuit 535 has highimpedance with respect to the second switching circuit 536, and theimpedance between the second input terminal 566 and the second outputterminal 568 of the second switching circuit 536 becomes low. As aresult, the receiving signal input from the second antenna terminal 503is outputted from the receiving terminal 504 but is not output from theother terminals.

As described above, the antenna switching device in Example 5 accordingto the present invention has the same function as that of Example 1, andconsumes almost no power.

Various other modifications will be apparent to and can be readily madeby those skilled in the art without departing from the scope and spiritof this invention. Accordingly, it is not intended that the scope of theclaims appended hereto be limited to the description as set forthherein, but rather that the claims be broadly construed.

What is claimed is:
 1. An antenna switching device comprising:atransmitting terminal for receiving a transmitting signal; a firstantenna terminal connected to a first antenna; a second antenna terminalconnected to a second antenna; a receiving terminal for outputtingreceiving signals received at the first antenna terminal and the secondantenna terminal; and selecting means for selecting one mode among afirst mode for outputting a transmitting signal from the transmittingterminal to the first antenna terminal, a second mode for outputting thereceiving signal from the first antenna terminal to the receivingterminal, and a third mode for outputting the receiving signal from thesecond antenna terminal to the receiving terminal; wherein the first,second and third modes are exclusive to each other, and the selectingmeans cannot select another mode in which the transmitting signal fromthe transmitting terminal is outputted to the second antenna terminaland the receiving terminal is not connected to the transmittingterminal.
 2. An antenna switching device according to claim 1, whereinthe selecting means comprises:a first switching circuit having a firstinput terminal electrically connected to the transmitting terminal, anda first output terminal electrically connected to the first antennaterminal; a second switching circuit having a second input terminalelectrically connected to the first antenna terminal, and a secondoutput terminal electrically connected to the receiving terminal; and athird switching circuit having a third input terminal electrically toconnected to the second antenna terminal, and a third output terminalelectrically connected to the receiving terminal; wherein the selectingmeans does not include another switching circuit connected to thetransmitting terminal and the second antenna terminal.
 3. An antennaswitching device according to claim 2, wherein, when the selecting meansselects the first mode, an electrical conduction is brought between thefirst input terminal and the first output terminal of the firstswitching circuit, and the second input terminal of the second switchingcircuit is electrically disconnected from the second output terminalthereof,when the selecting means selects the second mode, the firstinput terminal of the first switching circuit is electricallydisconnected from the first output terminal thereof, an electricalconduction is brought between the second input terminal and the secondoutput terminal of the second switching circuit, and the third inputterminal of the third switching circuit is electrically disconnectedfrom the third output terminal thereof, and when the selecting meansselects the third mode, the second input terminal of the secondswitching circuit is electrically disconnected from the second outputterminal thereof, and an electrical conduction is brought between thethird input terminal of the third switching circuit and the third outputterminal thereof.
 4. An antenna switching device according to claim 2,comprising:a low-pass filter electrically connected between thetransmitting terminal and the first switching circuit, for selectivelyallowing only a signal component of the transmitting signal having afrequency lower than a predetermined frequency to pass therethrough; anda reception filter electrically connected between the second switchingcircuit and the receiving terminal, for allowing only a signal componentof the receiving signal having a frequency within a predeterminedfrequency band to pass therethrough.
 5. An antenna switching deviceaccording to claim 4, wherein the reception filter is a band-pass filterwith attenuation poles.
 6. An antenna switching device according toclaim 2, wherein the first switching circuit has a first PIN diodeconnected between the first input terminal and the first outputterminal, and the third switching circuit has a second PIN diodeconnected between the third input terminal and the third outputterminal.
 7. An antenna switching device according to claim 6, whereinthe first switching circuit has a series circuit of an inductor and acapacitor, connected in parallel with the first PIN diode between thefirst input terminal and the first output terminal, and the thirdswitching circuit has a series circuit of an inductor and a capacitor,connected in parallel with the second PIN diode between the third inputterminal and the third output terminal.
 8. An antenna switching deviceaccording to claim 2, wherein the third switching circuit has an FET(field effect transistor), and a drain terminal and a source terminal ofthe FET are electrically connected between the third input terminal andthe third output terminal.
 9. An antenna switching device according toclaim 8, wherein the third switching circuit has an inductor, one end ofthe inductor is electrically connected to the drain terminal of the FET,and another end of the inductor is electrically connected to the sourceterminal of the FET.
 10. An antenna switching device according to claim2, wherein the second switching circuit has first and second inductorsconnected between the second input terminal and the second outputterminal, a first capacitor connected between the second input terminaland a ground, a second capacitor connected between the second outputterminal and the ground, and a PIN diode connected between the groundand a contact point between the first inductor and the second inductor.11. An antenna switching device according to claim 2, wherein the secondswitching circuit has a first quarter-wave transmission section, asecond quarter-wave transmission section, and a PIN diode, one end ofthe first quarter-wave transmission section is electrically connected tothe second input terminal, one end of the PIN diode and one end of thesecond quarter-wave transmission section are electrically connected toanother end of the first quarter-wave transmission section, another endof the PIN diode is grounded, and another end of the second quarter-wavetransmission section is electrically connected to the second outputterminal.
 12. An antenna switching device according to claim 2, whereinthe second switching circuit has a series circuit of an inductor and aPIN diode, electrically connected between the second input terminal andthe second output terminal, and a capacitor electrically connectedbetween the second input terminal and the second output terminal andconnected in parallel with the series circuit.
 13. An antenna switchingdevice according to claim 2, wherein the first switching circuit has aPIN diode, and the second and third switching circuits have FETs.
 14. Anantenna switching device according to claim 2, wherein the secondswitching circuit has an inductor, a capacitor, a first FET, and secondFET, one end of the inductor and one end of the capacitor arerespectively electrically connected to the second input terminal,another end of the capacitor is grounded, another end of the inductor iselectrically connected to a drain terminal of the first FET and a drainterminal of the second FET, a source terminal of the first FET isgrounded, and a source terminal of the second FET is electricallyconnected to the second output terminal.
 15. An antenna switching deviceaccording to claim 2, wherein the second switching circuit has aquarter-wave transmission section, a first FET, and a second FET, oneend of the quarter-wave transmission section is electrically connectedto the second input terminal, another end of the quarter-wavetransmission section is electrically connected to a drain terminal ofthe first FET and a drain terminal of the second FET, a source terminalof the first FET is grounded, and a source terminal of the second FET iselectrically connected to the second output terminal.
 16. An antennaswitching device according to claim 2, wherein the third switchingcircuit has a first FET and a second FET, a drain terminal of the firstFET and a drain terminal of the second FET are electrically connected tothe third input terminal, a source terminal of the first FET isgrounded, and the third output terminal is electrically connected to asource terminal of the second FET.
 17. An antenna switching deviceaccording to claim 2, wherein the third switching circuit has a firstFET, a second FET, and a resistor having resistance nearly equal to amatched load, a drain terminal of the first FET and a drain terminal ofthe second FET are electrically connected to the third input terminal, asource terminal of the first FET is connected to one end of theresistor, another end of the resistor is grounded, and a source terminalof the second FET is electrically connected to the third outputterminal.
 18. An antenna switching device according to claim 2, whereinthe third switching circuit has a first FET and a second FET, resistancebetween a drain terminal and a source terminal of the first FET in an ONstate is nearly equal to a matched load, the drain terminal of the firstFET and a drain terminal of the second FET are electrically connected tothe third input terminal, the source terminal of the first FET isgrounded, and a source terminal of the second FET is electricallyconnected to the third output terminal.
 19. An antenna switching deviceaccording to claim 1, comprising: a transmission filter electricallyconnected between the transmitting terminal and the first antennaterminal; and a reception filter having a circuit portion electricallyconnected between the second antenna terminal and the receiving terminaland having a dielectric resonator electrically connected to thereceiving terminal.
 20. An antenna switching device according to claim19, comprising a substrate having a first portion on which thetransmission filter is formed, a second portion on which the circuitportion of the reception filter is formed, and a third portion on whichthe dielectric resonator of the reception filter is formed,wherein thethird portion of the substrate is interposed between the first portionand the second portion.
 21. An antenna switching device comprising:atransmitting terminal for receiving a transmitting signal; a firstantenna terminal connected to a first antenna; a second antenna terminalconnected to a second antenna; a receiving terminal for outputtingreceiving signals received at the first antenna terminal and the secondantenna terminal; selecting means for selecting one mode among a firstmode for outputting the receiving signal from the first antenna terminalto the receiving terminal, and a second mode for outputting thereceiving signal from the second antenna terminal to the receivingterminal, said selecting means having a first switching circuitincluding a first input terminal electrically connected to the firstantenna terminal and a first output terminal electrically connected tothe receiving terminal, and a second switching circuit including asecond input terminal electrically connected to the second antennaterminal and a second output terminal electrically connected to thereceiving terminal; and is a transmission filter electrically connectedbetween the transmitting terminal and the first antenna terminal, forallowing a signal component having a frequency within a transmittingfrequency band to pass there through and having a high impedance withrespect to a signal component having a frequency within a receivingfrequency band; wherein the first and second modes are exclusive to eachother, wherein the first switching circuit is provided between the firstantenna terminal and the receiving terminal, and the second switchingcircuit is provided between the second antenna terminal and thereceiving terminal; and wherein no switching circuit is included betweenthe transmitting terminal and the first antenna terminal.
 22. An antennaswitching device according to claim 21, wherein the second switchingcircuit has a first FET, a second FET, and a resistor having resistancenearly equal to matched load, a drain terminal of the first FET and adrain terminal of the second FET are electrically connected to thesecond input terminal, one end of the resistor is connected to a sourceterminal of the first FET, another end of the resistor is grounded, anda source terminal of the second FET is electrically connected to thesecond output terminal.
 23. An antenna switching device according toclaim 21, comprising a reception filter electrically connected betweenthe first switching circuit and the receiving terminal.
 24. An antennaswitching device according to claim 23, wherein the reception filterselectively allows a signal component of the receiving signal having afrequency within a predetermined frequency band to pass therethrough.25. An antenna switching device according to claim 24, wherein thereception filter is a band-pass filter with attenuation poles.
 26. Anantenna switching device according to claim 21, wherein the transmissionfilter further has a low-pass characteristic.
 27. An antenna switchingdevice according to claim 26, wherein the transmission filter has aband-stop filter and a low-pass filter.
 28. An antenna switching deviceaccording to claim 21, wherein the second switching circuit has a PINdiode electrically connected between the second input terminal and thesecond output terminal.
 29. An antenna switching device according toclaim 28, wherein the second switching circuit has a series circuit ofan inductor and a capacitor, electrically connected between the secondinput terminal and the second output terminal, and the PIN diode isconnected in parallel with the series circuit.
 30. An antenna switchingdevice according to claim 21, wherein the second switching circuit has aFET having a drain terminal and a source terminal electrically connectedbetween the second input terminal and the second output terminal, and acontrol signal for controlling the FET is input to a gate terminal ofthe FET.
 31. An antenna switching device according to claim 30, whereinthe second switching circuit has an inductor, one end of which isconnected to the drain terminal of the FET and another end of which isconnected to the source terminal of the FET.
 32. An antenna switchingdevice according to claim 21, wherein the first switching circuit has afirst inductor, a second inductor, a first capacitor, a secondcapacitor, and a PIN diode, one end of the first inductor and one end ofthe first capacitor are electrically connected to the first inputterminal, another end of the first capacitor is grounded, one end of thePIN diode and one end of the second inductor are electrically connectedto another end of the first inductor, another end of the PIN diode isgrounded, another end of the second inductor and one end of the secondcapacitor are electrically connected to the first output terminal, andanother end of the second capacitor is grounded.
 33. An antennaswitching device according to claim 21, wherein the second switchingcircuit has a first FET and a second FET, resistance between a drainterminal and a source terminal of the first FET in an ON state is nearlyequal to a matched load, the drain terminal of the first FET end a drainterminal of the second FET are electrically connected to the first inputterminal, the source terminal of the first FET is grounded, and a sourceterminal of the second FET is electrical connected to the second outputterminal.
 34. An antenna switching device according to claim 21, whereinthe first switching circuit has a series circuit of an inductor and aPIN diode, electrically connected between the first input terminal andthe first output terminal, and a capacitor electrically connectedbetween the first input terminal and the first output terminal andconnected in parallel with the series circuit.
 35. An antenna switchingdevice according to claim 21, wherein the first switching circuit has aninductor, a capacitor, a first FET, and a second FET, one end of theinductor and one end of the capacitor are electrically connected to thefirst input terminal, another end of the capacitor is grounded, a drainterminal of the first FET and a drain terminal of the second FET areelectrically connected to another end of the inductor, a source terminalof the first FET is grounded, and a source terminal of the second FET iselectrically connected to the first output terminal.
 36. An antennaswitching device according to claim 21, wherein the first switchingcircuit has a quarter-wave transmission section, a first FET, a secondFET, one end of the quarter-wave transmission section is electricallyconnected to the first input terminal, another end of the quarter-wavetransmission section is electrically connected to drain terminal of thefirst FET and a drain terminal of the second FET, a source terminal ofthe first FET is grounded, and a source terminal of the second FET iselectrically connected to the first output terminal.
 37. An antennaswitching device according to claim 21, wherein the second switchingcircuit has a first FET and a second FET, a drain terminal of the firstFET and a drain terminal of the second FET are electrically connected tothe second input terminal, a source terminal of the first FET isgrounded, and a source terminal of the second FET is electricallyconnected to the second output terminal.